Spatially resolved optical spectroscopy on natural quantum dots

We report about optical experiments on (i) natural quantum dots formed by well width fluctuations in narrow GaAs quantum wells and on (ii) self-assembled InGaAs quantum dots in GaAs. Contributions from a single natural quantum dot are isolated by spatially resolved spectroscopy and resonant charge injection in an electric-field-tunable coupled quantum well structure. In optical experiments the excitonic and bi-excitonic ground states are identified and growth related interface problems are analyzed. Spectral linewidths down to 50 meV allow thereby a complete separation of the individual levels and Zeeman sub-levels of the dot. The natural quantum dots with their fully resolved Zeeman levels are used as local probes to monitor the spin orientation and the creation of spin-polarized excitons in coupled quantum well structures. With cathodoluminescence excited by a scanning tunneling microscope tip and a special sample design we are able to excite selectively the ground state of a single self-assembled InGaAs quantum dot.